Packaging material with a foamed polyolefin layer

ABSTRACT

The invention relates to a multilayer film comprising the following sequence of layers:
         A) a basis layer of polyolefin foam,   B) a layer based on at least one of the polyolefins of the foam layer A),   C) optionally a tie layer based on a polyolefin,   D) optionally an adhesive layer,   E) optionally a gas- and/or flavortight barrier layer,   F) an adhesive layer,   G) an optionally sealable and/or peelable surface layer,
 
whereby the total thickness of layers A) and B) ranges from 0.5 to 2 mm and the thickness of layer B) ranges from ⅙ to ½ of the thickness of layer A).

This is a 371 of PCT/EP99/06191 filed 24 Aug. 1999 (international filingdate).

The invention is in the field of packaging industry and relates topackaging material comprising a polyolefin foam layer having on one ofits surfaces further layers.

BACKGROUND OF THE INVENTION

Packaging materials with a foam layer of e.g. polypropylene in the formof quasi endless rolled webs are used for producing thermoformedself-supporting packaging items as e.g. trays for food. Therefore thesekinds of packaging materials are formed on so called “form-fill-seal”packaging machines (“FFS-machines”) to packaging items. According tothis method the web of packaging material is fed into the machine inwhich it is formed into a web of trays which trays in the same machineare filled and sealed with a preferable transparent sealing film. Thenusually the trays are separated from each other.

In the known packaging materials the polypropylene foam layer iscarrying on at least one of its surfaces further layers. Depending onthe composition and the thickness of these layers these layers may alsoserve as further means for increasing the stiffness as well asprotection of the foam layer surface or as gas and/or aroma barrierand/or as surface layer for bounding the packaging tray and a preferabletransparent lidding film.

Packaging materials with a polypropylene foam layer for producing traysby thermoforming are disclosed e.g. in EP-A-0 570 222. These materialscomprise a foam layer and a multilayer film which multilayer film mayinclude a barrier layer rendering the packaging material gas- and/oraroma tight. The properties of this multilayer packaging material needsome improvement as far as the use of this material as self-supportingpackaging items is concerned.

Therefore it is has been proposed to bond the foam sheet with themultilayer film by means of a bonding layer of a polyolefin based on themain monomer of the polyolefin of the foam layer (WO 96/25290). Thisresults in an excellent adhesion between the laminate and the foamlayer. However the packaging items especially self-supporting onesproduced from this known multilayer film composition are not rigidenough for some applications.

Therefore there is a need to improve the stiffness of such packagingitems, preferably trays, without increasing the thickness of thepackaging material. Besides improving the mechanical properties of thepackaging material there is also a demand to provide a packagingmaterial suitable for thermoforming over a broader temperature range inorder to achieve a faster production of packaging items.

SUMMARY OF THE INVENTION

According to the present invention, this object is achieved by providinga multilayer film comprising the following sequence of layers:

-   A) a base layer of a polyolefin foam,-   B) a layer based on at least one polyolefin of the foam layer A),-   C) optionally a tie layer composed of a polyolefin, which is    preferably based on the monomer that is the main monomer of the    polyolefin of the foam layer A),-   D) optionally an adhesive layer,-   E) optionally a gas- and/or flavortight barrier layer,-   F) an adhesive layer,-   G) an optionally sealable and/or peelable surface layer,    whereby the total thickness of layers A) and B) ranges from 0.5 to 2    mm and the thickness of layer B) ranges from ⅙ to ½ of the thickness    of layer A).

DETAILED DESCRIPTION

Preferably the total thickness of the layers A) and B) ranges from 0.6to 1.4 mm and the thickness of layer B) is from ⅙ to ⅓ of the thicknessof layer A).

The foam layer A) consists of at least one foamed polyolefin. Forproducing packaging items such as trays applicable in food packagingfoamed propylene- homo- and/or -copolymers are used advantageously assuch materials are self-supporting with lower thickness and lowerdensity. A mixture of polyolefins can also be used for the manufactureof the foam layer. A mixture of polypropylene with long chain branching(high-melt-strength polypropylene) and a propylene ethylene copolymer(e.g. heterophasic propylene ethylene blockcopolymer) can preferably beused. Particularly suitable is a mixture of a polypropylene with longchain branching and a melt-index (MFI) in the range of 1.4 to 4.2 g/10min and a heterophasic propylene ethylene block copolymer in a mixingratio of 1:1.

The polyolefin of layer A) can be foamed by addition of solid, liquidand/or gaseous blowing agents in known amounts preferable in an amountof 0.5 to 3 wt. %. The relevant disclosure of EP-A-0 570 222 is herewithcited as referential disclosure for the present application.

Foam layers of polyolefins, preferable of polypropylene, optionallyadmixed with copolymers of olefins, preferable with propylene ethylenecopolymers, used for the production of packaging materials preferablyhave a density of 0.1 to 0.8 g/cm³, preferably 0.25 to 0.5 g/cm³, and acell count of 75 to 300 cells per mm³. Density and cell count can beinfluenced by the process conditions like, e.g., extrusion temperatureor other process condition during the preferred method for producingfoam layers, namely by extrusion and expansion.

Layer B) of compact polyolefin essentially consists of a polyolefin ofthe foamed layer A). In case the basis layer A) is made of a foamedpolypropylene or a foamed mixture of polypropylene and propyleneethylene copolymer the compact polyolefin layer B) is preferably madeeither of polypropylene or a propylene ethylene copolymer. Particularlypreferred a heterophasic propylene ethylene blockcopolymer is used. Themelt index (MFI) of the polyolefin used for the manufacture of layer B)ranges preferably from 1.8 to 5.5 g/10 min. The thickness of layer B) is⅙ to ½, most preferably ⅙ to ⅓ of the thickness of layer A).

Layer C) is present when layers D) to G) are pre-produced by coextrusionpreferable by blown film coextrusion and to be combined with the otherlayers. Layer C) is based on a polyolefin which is preferably made froma monomer which is also the main monomer of the polyolefin of the foamlayer A). In case layer A) consists of a foamed polypropylene andoptionally a propylene ethylene copolymer, layer C) can be made ofpolypropylene optionally grafted with maleic acid anhydride. Copolymersof ethylene vinylacetate can also be used as materials for layers C).The thickness of layer C) ranges preferably from 5 to 25, mostpreferably from 8 to 15 μm.

In case the inventive multilayer film should have a low gaspermeability, i.e. a low permeability of oxygen and moisture and a highflavor tightness it comprises a barrier layer E). This barrier layer ispreferably made of ethylene vinyl alcohol copolymer having an ethylenecontent of 32 to 45 Mol %, preferably 35 to 42 Mol %.

The barrier layer E) is bonded on its respective surfaces to the tielayer C) or to the surface protecting layer G) by means of an adhesivelayer D) or F), respectively. The material used for this is preferably apropylene copolymer or a polyethylene grafted with maleic acidanhydride, respectively.

The surface protecting layer G) is preferably sealable or peelable.Thus, preferably low density polyethylene (LDPE) having a melt index(MFI) in the range of 0.5 to 8.0 g/10 min, preferably of 1 to 5 g/10 min(2.16 kg, 190° C. measured according to ASTM D 1238) or a ionomerpolymer like, e.g., a copolymer of an α-olefin and an ethylenicallyunsatured monomer with a carboxylic group, which are present in anamount of 20 to 100% by weight as metal salt, preferably Zn-salt; orethylene-vinylacetate copolymers with a vinylacetate content of 3 to 10%by weight, preferably 4 to 6% by weight is used for the manufacture ofthe sealable layer.

According to a particularly preferred embodiment the sealing layer isalso peelable. The sealing layer then preferably comprises a mixture ofLDPE and polybutylene (PB). To this end the mixture comprises 15 to 30weight %, preferably 20 to 28 weight % of polybutylene. The melt index(MFI) of the polybutylene is preferably in the range of 0.3 to 2.0 g/10min (190° C. and 2.16 kg, ASTM 1238).

Preferably the thickness of the surface layer is in the range of 10 to50 μm, preferably 15 to 30 μm.

When using LDPE as a sealing layer material and the inventive multilayerfilm comprises a barrier layer, usually an adhesive layer is requiredfor connecting the barrier layer and the sealing layer, unless ethylenevinyl alcohol is used as material for the barrier layer. As adhesivematerial a polyolefin, preferably a polyethylene grafted with maleicacid anhydride may be used.

A LDPE and LLDPE mixture can also be used in a ratio from 3:1 to 4:1 asadhesive material. The thickness of the respective adhesive layer rangesfrom 2 to 8 μm, preferably from 3 to 6 μm.

The surface layer G) may contain usual and known lubricating agents andanti-blocking agents, as, for example, erucic acid amide,polyalkylsiloxane, such as for example polydimethyl siloxane and/orSiO₂. All or individual layers can contain stabilizers and furtheradditives of known kind.

Furthermore layer B) can contain 0.5 to 2% by weight of a white pigmentlike, e.g., kaolin, calcium carbonate, talcum or titanium dioxide ormixtures thereof. These inorganic pigments are added to the polymer ofwhich layer B) is made, preferably in form of a master batch containing30 to 50% by weight of pigments. Layer B) can also be composed of up to30% by weight of recycled inventive multilayer film material.

The multilayer film according to the invention is preferably produced bythe usual blown film coextrusion or cast film coextrusion processes asfar as the production of the sequence of layers C) to G) is concernedand preferably by an extrusion lamination step for combining with thepolyolefin foam layer A). Thereby the foam layer A) and the multilayerfilm consisting of layers C) to G) are guided together in such a waythat layer B) is extruded between them. Immediately after the extrusiona sufficiently high pressure is applied to the so produced laminate, sothat layers A) to G) are sufficiently bonded to one another.

However it is also possible to produce the inventive multilayer films bycoextrusion, whereby the foam layer A) is simultaneously coextruded withthe other layers, optionally without layer C), and by expanding layerA).

The inventive multilayer films have an excellent thermoformability on socalled FFS-machines (form-fill and sealing machines) and thus can beformed to packaging items preferable to trays by deep drawing. Thesetrays can also be filled and sealed on the same machine. It wassurprising to find out that the multilayer films according to thepresent invention cannot only be excellently processed on theseFFS-machines but can also be formed to packaging items, particularlydeep drawn trays with a higher speed when compared to known multilayerpackaging materials.

The inventive multilayer films show an unexpected improvement of themechanical properties such as resistance to breaking and stiffness ofthe E-modulus, although their total thickness is reduced compared toknown comparable packaging materials. This considerable improvement isachieved without any increase of the thickness of the multilayer filmswhich is being required as for the films known from the prior art.Surprisingly the mechanical properties are even improved although thetotal thickness of the multilayer packaging material is reduced.

A further object of the invention is therefore the use of the multilayerfilm according to the present invention as packaging material,preferably on form-, fill- and sealing machines (FFS-machines) forpackaging of food, especially for packaging of perishable food such asmeat, ham or sliced meat. Because packaging on FFS-machines is veryoften carried out discontinuously in order to pack differnt goods, thepackaging material must show a relatively broad so called thermoformingwindow in which the thermo-forming of the film can occur. This isachieved by providing the inventive packaging material, which hasexcellent thermo-formability, especially deep drawing qualities.

The thermo-formed, preferably deep-drawn, packaging items, such as,e.g., deep drawn trays, which can be produced from the multilayermaterials according to the present invention can be sealed with knownlidding film after they have been filled. Suitable lidding films aremultilayer films, preferably from polyethyleneterephthalate/SiO_(x)/adhesive layer/low density polyethylene or frompolyethylene terephthalate/adhesive layer/polyethylene/adhesivelayer/ethylene vinyl alcohol copolymer/adhesivel layer/polyethylene.

Of the above mentioned lidding films, the first-mentioned multilayerfilms are particularly suitable. Therefore the biaxially orientedpolyethylene terephthalate is coated with SiO_(x) by plasma vacuotechnique. In a following step the prefabricated polyethylene film islaminated thereto with an adhesive. Such a lidding film is characterizedby a excellent transparency and a high breaking resistance. Thepackaging items having such a lidding film show an excellent gas barrierproperty.

In the following examples the E-modulus is determined according to DIN43457 in the machine direction and transverse to this direction.Furthermore he elongation at break in the machine direction andtransverse thereto is determined according to DIN 53455.

EXAMPLES Example 1

An inventive multilayer film is produced with the following sequence oflayers:

-   Layer A) Foam layer with a density of 0.3 g/cm³ and a cell count of    120 cells per mm³ made of a 50:50 wt % mixture of a polypropylene    with long chain branching (high melt strength polypropylene) and a    heterophasic propylene ethylene blockcopolymer,-   Layer B) consists of 100% by weight of heterophasic propylene    ethylene block copolymer like the one used for foam layer layer A),-   Layer C) consists of polypropylene,-   Layer D) consists of an adhesive based on polypropylene grafted with    maleic acid anhydride,-   Layer E) as a gas barrier layer consisting of an ethylene    vinylalcohol copolymer,-   Layer F) as adhesive layer having the same composition as Layer D),-   Layer G) as a sealing layer based on low density polyethylene.

The multilayer film is produced by co-extrusion.

The thickness of the individual layers are set forth in table 1.

Table 1:

TABLE 1 Layer A) B) C) D) E) F) G) Total Thickness in μm 800 160 15 5 55 20 1010

The mechanical properties of said fim are set forth in table 3.

Comparative Example

A multilayer film having an identical layer sequence as well asidentical layer compositions, but different thicknesses of layers isproduced corresponding to the multilayer-film according to example 1.

The thickness of the individual layers of this multilayer film is setforth in table 2:

Table 2:

TABLE 2 Layer A) B) C) D) E) F) G) Total Thickness in μm 1160 40 15 5 55 20 1250

The mechanical properties of this multilayer film are set forth in table3:

Table 3:

TABLE 3 comp. unit ex. 1 ex E-modulus (machine direction) N/mm² 788 564E-modulus (transverse direction) N/mm² 584 426 Elongation at break(machine direction) N/mm² 10.2 6.9 Elongation at break (transversedirection) N/mm² 8.4 5.6

As can be seen from table 3, the inventive multilayer films have farsuperior mechanical values, even though the total thickness of the filmis less than of the known packaging materials. Therefore the inventivemultilayer films are by far superior for the production ofself-supporting packaging, particularly deep drawn trays.

1. A multilayer film comprising the following sequence of layers: A) abase layer of foamed propylene homopolymers, copolymer or mixturesthereof, B) a layer comprising at least one of the propylenehomopolymers or copolymers of the foam layer A, C) optionally a tielayer based on a polyolefin D) optionally an adhesive layer E)optionally a gas barrier layer, a flavortight barrier layer, or both, F)an adhesive layer, G) an optionally sealable or peelable surface layer,whereby the total thickness of layers A and B ranges from 0.5 to 2 mmand the thickness of layer B ranges from ⅙ to ½ of the thickness oflayer A.
 2. A multilayer film according to claim 1, wherein the totalthickness of layers A and B ranges from 0.6 to 1.4 mm.
 3. A multilayerfilm according to claim 1 wherein the thickness of layer B ranges from ⅙to ⅓ of the thickness of layer A.
 4. A multilayer film according toclaim 1, wherein layer A is made of a foamed mixture of polypropylenewith long chain branching and a propylene-ethylene-blockcopolymer.
 5. Amultilayer film according to claim 1, wherein layer B is made ofpolypropylene or a propylene-ethylene-copolymer.
 6. A multilayer filmaccording to claim 1, wherein layer C is made of a polyolefin based on amonomer which is present in a predominant amount in the polyolefins offoam layer A.
 7. A multilayer film according to claim 6, wherein thepolyolefin is polypropylene.
 8. A multilayer film according to claim 1,wherein layer E is present and is made of ethylene-vinylalcoholcopolymer.
 9. A multilayer film according to claim 1, wherein layer G ismade of a sealable polymer and optionally contains additives.
 10. Amultilayer film according to claim 9, wherein layer G is made of a lowdensity polyethylene or an ionomer.
 11. A multilayer film according toclaim 1, wherein layer G is made of a peelable polymer and optionallycontains additives.
 12. A multilayer film according to claim 11, whereinlayer G is made of a mixture of low density polyethylene and apolybutylene.
 13. A multilayer film according to claim 1, wherein thetotal thickness of layers C to G ranges from 20 to 70 μm.
 14. Amultilayer film according to claim 13, wherein the total thickness is 30to 50 μm.
 15. A packaging material comprising the multilayer film ofclaim
 1. 16. A packaging item made of a film according to claim
 1. 17. Apackaging item according to claim 16, wherein said packaging item is apackaging tray.
 18. A method for packaging meat, sausage or cheese whichcomprises packaging said meat, sausage or cheese in a multilayer filmaccording to claim
 1. 19. A method of packaging material on form-, fill-and sealing packaging machines which comprises packaging said materialin a film of claim
 1. 20. A food packaging container which comprises themultilayer film of claim 1.